saw
Whenever I saw by hand I end up with edges that aren't straight. This includes both the long edge and short edge, or horizontally and vertically.
What is the key to sawing straight by hand?
Update 1
I've had the same wonky results with a crosscut saw, coping saw, and backsaw.
Yes, you might want the highest precision possible - for example when you build furniture which benefits greatly from precise cuts. The most convenient way would be to draw the line that will signify the edge of the piece you want cut so that the blade cuts the line and whatever material is on the far side of the raw board. Something like this:
|the detail you want|line|remaining material|
| |blade cuts here|
This is quite easily achieved with power saws even without a guide and will be more of a challenge with hand saws.
Rent a mitre saw. It is the quickest way.
You will very quickly regret attempting to hand saw a hardwood flooring installation. Especially when you need to do a rip cut.
"If hand-sawing is out, are there any miter box attachments for circular saws? Googling has only turned up"
You could use a circular saw and/or jigsaw. If you wanted an inexpensive and quick "plug and play" solution, there are plenty of saw guides you can buy for square or angled cuts.
Note that especially for angled cuts with a saw guide, accurately lining up your cut line with the saw blade can be a challenge. Even more so for someone not experienced doing this.
If I had to install a hardwood floor tomorrow and all of my mitre saws were broken and no rental stores had any available, I would make myself some sort of cutting station. Similar to this design:
In the image above the coloured end piece is cut off exactly in line with the edge of the board above it.
If you stack some wood on the bottom piece the height of a hardwood plank, and leave a slot the exact width of a hardwood plank- You could mark a cut line on the plank, slide it in to the slot, lining it up with the cutting edge and make quick and accurate cuts, similar to speed and accuracy of a mitre saw.
Best Answer
I can think of two possibilities: 1) you're beginning your cut with the saw out of alignment, or 2) the saw teeth are not set correctly. ("Set" is the distance of the tip of each saw tooth from the center. If you look, you'll notice that the saw teeth are bent slightly away from the saw plate in alternating directions).
Let's assume you have already worked out your body geometry and you're holding the saw straight to begin with. It's possible that the saw teeth are set more on one side than the other. This can cause a saw to drift off the intended cut line.